(1) Field of the Invention
The present invention relates to a non-dispersive infrared analyzer for detecting concentration of a preselected gas in a sample gas using an absorption band of infrared light which is characteristic of the preselected gas.
(2) Description of the Related Art
Non-dispersive infrared analyzers are now widely used in measuring gas concentration of various types of exhaust gases in order to monitor the exhausts for air pollution, for measuring gas concentration in industrial processes, etc. One such non-dispersive infrared analyzer, as an example, is described in U.S. Pat. No. 4,355,233 by Warnke et al.
A prior art dual beam non-dispersive infrared analyzer is illustrated herein in FIG. 5. FIG. 6 depicts a rotating chopper thereof. In the FIG. 5, item 1 indicates an infrared source, item 3 is a cylindrical sample cell, item 5 is an amplifier, item 6 is an electric circuit, item 7 is a motor, item 8 is an axle connected to the motor, item 9 is a position sensor, item 101 is a cylindrical reference cell, item 102 is a light collector, item 103 is a rotating chopper having a disc shape, and item 104 is a pneumatic detector. Item 103a of FIG. 6 is a first aperture located at an outer track of the rotating chopper, and 103b is a second aperture located at an inner track of the rotating chopper.
The cylindrical walls of sample cell 3 and reference cell 101 are typically formed of aluminum. Both ends of the cylindrical cells 3 and 101 are equipped with cell windows through which an infrared light beam passes. The cell windows are made of plate which is transparent to infrared light, such as calcium fluoride, CaF.sub.2.
In practice, the infrared source 1 emits a wide spectrum of infrared radiation. The rotating chopper 103 is operated to interrupt the infrared light by periodical rotation, for example, one rotation per one second (1 Hz). The infrared radiation travels to the reference cell 101 through the first aperture 103a and to the sample cell 3 through the second aperture 103b, respectively. In the sample cell 3, a specimen gas which includes the gas to be measured, flows through. The reference cell 101 is an airtight cell and is filled with a gas which does not absorb infrared energy, such as N.sub.2 gas. The beam, having traveled through the sample cell 3 or the reference cell 101, reaches detector 104 which has selective sensitivity of the infrared radiation band set to correspond with the absorption wavelength of a gas to be measured.
Detector 104 can be a pneumatic detector, which is a one type of such detector. Detector chamber 104a and 104b are generally filled with the same kind of gas as that to be measured, and are separated by a diaphragm 104c. Diaphragm 104c is a very thin sheet of a conductive material such as titanium foil which serves as a variable plate of a capacitor. Disposed next to diaphragm 104c is an electrode 104d which serves as a fixed plate of the capacitor. Infrared energy within the absorption band, characteristic of the gas to be measured, which has not been absorbed by the component gas in sample cell 3, will be absorbed by the same gas in the chamber 104a. The gas in the chamber 104a will become heated to a greater extent than that in the chamber 104b. The unequal heating in the chambers 104a and 104b will produce a pressure difference, causing deflection of diaphragm 104c which, in turn, will vary the capacitance established between electrode 104d and diaphragm 104c.
Detector 104 generates an electric signal, which is then amplified by amplifier 5. The signal is fed to electrical circuit 6.
Rotating chopper 103 is connected to the motor by the axle 8, and rotates periodically. The infrared radiation emitted by the infrared source is exposed to the reference cell 103 and the sample cell 3 alternatively and periodically in a time-sharing manner through the first aperture 103a and the second aperture 103b of rotating chopper 103. Also, position sensor 9 which is placed near outer track of the chopper disk, senses the first aperture, and feeds the position signal to the electrical circuit 6. The electrical circuit determines whether the detector signal belongs to the sample signal generated by the beam traveled through the sample cell 3 or the reference signal generated by the beam traveled through the reference cell 101.
In order to eliminate an effect caused by the fluctuation of the intensity of emitted light and/or the fluctuation of the sensitivity of detector 104, caused by variables such as temperature, the sample signal is normalized using the reference signal. The sample signal is divided by the reference signal, and the normalized intensity ratio is obtained. Because the intensity ratio is proportional to the concentration of the gas to be measured in the sample cell, the electrical circuit calculates the concentration of the gas to be measured.
As mentioned above, a prior art dual beam non-dispersive infrared analyzer employs dual cells. Very often, the two cells will have different permeability of infrared light due to, for example, stains on cell window and/or on inner wall of the cell caused by constituent of sample gas. Also, the light collector may not be completely symmetrical for the two beams, which can introduce inconsistencies in the reading of the beams. Because the normalization of the sample signal by the reference signal is based on the hypothesis that the two beams passing through the cells are of equal character concerning infrared light, these differences or inconsistencies can render the output concentration result inaccurate. Thus, balance adjustment of the two beam passes is necessary.
Further, the structure of the infrared analyzer is complicated due to the dual beam structure. Moreover, since the infrared radiation goes through the relatively small aperture to travel to the cells in a time-sharing manner with the rotation of the chopper, the utilized portion of the emitted infrared radiation for the measurement is less than one fourth (1/4) of the emitted radiation by infrared source. In order to overcome this drawback, or minimize the effect thereof, cell inner walls of some of prior art apparatuses are plated with gold.
It is therefore an object of the present invention to provide a simple single beam, non-dispersive infrared analyzer having high accuracy.
It is another object of the present invention to provide compensation means to rectify a fluctuation of the overall sensitivity of the non-dispersive infrared analyzer.
It is further object of the present invention to provide an inexpensive non-dispersive infrared analyzer for measuring gas concentration in a sample gas stream.
It is further object of the present invention to provide an non-dispersive infrared analyzer with a desired bandpass filter utilizing an airtight cell filled with a preselected gas.
It is further object of the present invention to provide an non-dispersive infrared analyzer with increasing the utilizing portion of the infrared radiation emitted by the infrared source.